Membranes of the above kind are described in detail in, for example, EP-A-0 568 045, EP-A-0 168 783, EP-B-0 082 433; and WO 86/00028. These membranes are manufactured from polymeric synthetic materials, they have asymmetric structure with high diffusive permeability (clearance) and have water filtration capability with ultrafiltration in the range of low flux to high flux. In EP-A-0 305 787, a 3-layer structure membrane and filter with corresponding performance, is disclosed.
The membranes according to prior art are well performing, but still have some space for improvement and optimization. The areas of improvable properties are that the fibers are difficult to handle, they stick together and adhere to each other, which cause problems during manufacturing of dialysers, specifically when potting them in polyurethane (PUR). Further, the permeability of the fibers is still improvable. Thus, the diffusive permeability (clearance) for different molecular weight substances in the range of urea can be improved, as well as to a higher extent the permeability for substances with middle molecular weight range, like β2-M, factor D and others, but with low albumin permeability.
To achieve a high permeability for the substances with low and middle molecular weight on the one hand and on the other hand have a low permeability for albumin, is one of the requirements put on dialysis membranes. This characteristic is called “selectivity”. The selectivity of prior art membranes still needs to be improved.